The energy change of reactions (HT only)

AQA·Chemistry (Combined Science)·5.5.1.3·Trilogy — Higher

Calculating reaction energy change from bond energies

A chemical reaction breaks bonds in the reactants and forms new bonds in the products. Breaking bonds requires energy, while forming bonds releases energy. The overall energy change depends on which total is greater.

A bond energy is the energy needed to break one mole of a particular type of bond. It is usually given in $$\text{kJ mol}^{-1}$$. Forming that same amount of the bond releases an equal amount of energy.

To calculate the reaction energy change, add the bond energies for all the bonds broken, then add the bond energies for all the new bonds formed. Take account of the number of each type of bond.

Reaction energy change=energy needed to break bonds−energy released when bonds form\text{Reaction energy change} = \text{energy needed to break bonds} - \text{energy released when bonds form}

If bond formation releases more energy than bond breaking requires, the result is negative and the reaction is exothermic overall. If bond breaking requires more energy than bond formation releases, the result is positive and the reaction is endothermic overall.

Bond-energy calculation: hydrogen and chlorine

Hydrogen reacts with chlorine to form hydrogen chloride:

HX2+ClX2→2 HCl\ce{H2 + Cl2 -> 2HCl}

Using bond energies of $$436\ \text{kJ mol}^{-1}$$ for $$\ce{H-H}$$, $$243\ \text{kJ mol}^{-1}$$ for $$\ce{Cl-Cl}$$ and $$431\ \text{kJ mol}^{-1}$$ for $$\ce{H-Cl}$$:

  • Bonds broken: one $$\ce{H-H}$$ and one $$\ce{Cl-Cl}$$ bond require a total of $$436 + 243 = 679\ \text{kJ}$$.
  • Bonds formed: two $$\ce{H-Cl}$$ bonds release a total of $$2 \times 431 = 862\ \text{kJ}$$.

For one mole of reaction as written, the energy change is:

ΔE=679−862=−183 kJ mol−1\Delta E = 679 - 862 = -183\ \text{kJ mol}^{-1}

The negative result shows that the reaction is exothermic: forming the new bonds releases more energy than is needed to break the original bonds.

Energy transfer in bond breaking and bond making

In a chemical reaction, atoms are rearranged: some existing bonds break and new bonds form. Both changes involve energy, but in opposite directions.

Breaking a bond requires energy. Energy must be supplied to overcome the forces holding the bonded atoms together. Forming a bond releases energy. The newly bonded arrangement is more stable than the separate atoms at that stage.

The overall energy change depends on the balance between these two transfers:

  • If forming product bonds releases more energy than is needed to break reactant bonds, the reaction releases energy overall.
  • If breaking reactant bonds requires more energy than is released when product bonds form, the reaction absorbs energy overall.

A reaction can therefore release energy overall even though energy must be supplied to break bonds. The energy released when new bonds form more than makes up for that input.